A magnetic random access memory (MRAM) is a memory for storing information through use of the magnetization direction of a magnetic layer. The MRAM has been developed due to its desired characteristics such as a low-voltage operation, nonvolatility, a theoretically infinite rewrite frequency, and a high-speed operation.
In recent years, there has been proposed a magnetic domain wall displacement MRAM in which the magnetization reversal of a recording layer is induced and controlled with a torque that acts between the spin of an electron and the magnetic moment localized in a magnetic substance. The magnetization reversal is hereinafter referred to as “current-driven magnetic domain wall displacement”. The magnetic domain wall displacement MRAM is disclosed in, for example, Patent Documents 1 to 5.
In the magnetic domain wall displacement MRAM, a magnetic domain wall displacement element is used as a memory element. FIGS. 1A and 1B are respectively a schematic plan view and a schematic sectional view of a typical magnetic domain wall displacement element. The magnetic domain wall displacement element includes a recording layer 10 in which a magnetic domain wall MW is displaced, and magnetic layers (first fixed layer 16 and second fixed layer 17) that are magnetically combined with the magnetization of the recording layer 10. The recording layer 10 includes a reversal region 13 in which the magnetization direction can be reversed and two fixed regions (first fixed region 11 and second fixed region 12) formed on both sides of the reversal region 13. The respective magnetization directions of the first fixed region 11 and the second fixed region 12 are fixed in directions opposite to each other. As a result, in the recording layer 10, the magnetic domain wall MW is formed at a boundary between any of the fixed regions and the reversal region 13. Note that, in FIGS. 1A and 1B, a read mechanism for reading the magnetization direction of the reversal region 13 is not shown.
In the magnetic domain wall displacement element, the magnetization direction of the reversal region 13, that is, the position of the magnetic domain wall MW is associated with stored data. The stored data is rewritten by displacing the magnetic domain wall MW so as to reverse the magnetization direction of the reversal region 13. In order to displace the magnetic domain wall, a write current is supplied in an in-plane direction in the recording layer 10. The magnetic domain wall MW is displaced in the recording layer 10 in accordance with the direction of conduction electrons carrying the write current.
Further, in Non Patent Document 1, it is theoretically suggested that, in the case where a perpendicular magnetization film having perpendicular magnetic anisotropy is used as a magnetization recording layer, the write current is reduced compared to the case of using an in-plane magnetization film having in-plane magnetic anisotropy. Thus, it is preferred that the perpendicular magnetization film be used as the magnetization recording layer (magnetic domain wall displacement layer) in the magnetic domain wall displacement MRAM.
Further, according to Non Patent Documents 2 and 3, there is reported a recent new technology for controlling the magnetization of a magnetic film by causing a current to flow through the magnetic film while applying an in-plane magnetic field thereto. This technology has advantages in that a magnetic layer for polarizing an electron is not required, and hence a structure is simple. The reversal phenomenon of magnetization is considered to be caused by either the Rashba field generated by a non-symmetrical three-layer structure formed of a heavy metal layer, a ferromagnetic layer, and an oxidized metal layer or the spin-current injection that occurs due to the spin Hall effect of the heavy metal layer, and has currently been discussed actively. The magnetization reversal is hereinafter referred to as “magnetization reversal induced by an in-plane magnetic field and an in-plane current”.
Note that, the direction in which magnetization is finally directed by the “magnetization reversal induced by an in-plane magnetic field and an in-plane current” depends on the kind and lamination order of the heavy metal layer. Specifically, in the case where the ferromagnetic layer is laminated on the heavy metal layer made of Pt or the like, the magnetization is finally directed in a direction of ZXIXH, with a unit vector Z in a Z-direction, an in-plane magnetic field H, and an in-plane current I being respectively vectors. In this case, the X represents an outer product. Further, in the case where the ferromagnetic layer is laminated under the heavy metal layer made of Pt or the like, the magnetization is finally directed in a direction of −ZXIXH. Further, in the case where the ferromagnetic layer is laminated on the heavy metal layer made of Ta or the like, the magnetization is finally directed in the direction of −ZXIXH. Further, in the case where the ferromagnetic layer is laminated under the heavy metal layer made of Ta or the like, the magnetization is finally directed in the direction of ZXIXH. In the description below, for simplicity, the case where the ferromagnetic layer is laminated on the heavy metal layer made of Pt or the like is described. However, the lamination can be appropriately modified or changed.